iceland spar

noun

1. a transparent variety of calcite that is double-refracting and is used as a polarizer.

noun

1. a pure transparent variety of calcite with double-refracting crystals used in making polarizing microscopes

1. A form of calcite that is transparent and causes light passing through it to refract in two directions (forming a double image of an object seen through it). Iceland spar occurs in perfect rhombohedrons and is used in optical instruments.